Ultrasonic transit-time flow measurement is well known for measuring fluid flow velocities through conduits. Moreover, such flow measurement is feasible without introducing mechanical obstructions to the flow. In addition to an advantage of such non-obstructiveness, ultrasonic flow measuring apparatus often offer a relatively low cost of installation and operation. This is in particular true for apparatus that are clamped to an outside of conduits guiding flows of fluids in operation. Numerous methods and apparatus for ultrasonic flow measurement have been proposed and patented since the 1950's. A general review of ultrasonic flow measurement is to be found in Lynnworth & Liu, Ultrasonics 44, 2006, pp. 1371-1378.
In an ultrasonic flow measuring apparatus, at least one pair of ultrasonic transducers are configured at upstream and downstream positions relative to each other. The pair of transducers alternately transmits and receives ultrasonic signals that propagate along at least one path in a fluid to be characterized. Transit times of upstream- and downstream-propagating signals can be used to compute a flow velocity of the fluid.
FIG. 1 is an illustration of an example of a conventional known type of ultrasonic transit-time flow measuring apparatus mounted to a conduit 10. The apparatus employs a fixed acoustic propagation path 20 having a spatial extent from a first ultrasonic transducer 30 (A) via a region of fluid 40 at an angle φ relative to an elongate axis of the conduit 10, to a second transducer 50 (B). Firstly, an ultrasonic signal is sent in a first direction from the first transducer 30 via the region of fluid 40 to the transducer 50 (B). Secondly, an ultrasonic signal is then transmitted in the opposite direction, from the second transducer 50 (B) via the region of fluid 40 to the first transducer 30 (A). Thus, two transit times are measured for the first and second directions, namely tu for upstream ultrasonic signal propagation, and td for downstream ultrasonic signal propagation. Assuming that the ultrasound signal velocity c is much greater than the fluid flow velocity v, namely v2<<c2, an inference of an axial flow velocity of the fluid in the region 40 can be derived from the transit times tu, td using Equation 1 (Eq. 1):
                    v        =                                                            c                2                            ⁢              tan              ⁢                                                          ⁢              φ                                      2              ⁢              D                                ⁢                      (                                          t                u                            -                              t                d                                      )                                              Eq        .                                  ⁢        1            wherein D is a distance between inner surfaces of the conduit 10, for example a diameter of the conduit 10 when it has a round profile. The sound velocity c in the fluid and the angle φ between the wall of the conduit 10 and the direction of ultrasound propagation along the path 20 are previously determined quantities. A derivation of Equation 1 (Eq. 1) can be found in U.S. Pat. No. 5,856,622.
It is conventional to employ a model which regards the two transducers 30, 50 as points and ultrasonic radiation propagating as rays through these points. The diameter D and a distance L between the transducers 30, 50 determine an acoustic propagation path for propagation of the ultrasonic radiation. The ultrasonic transducers 30, 50 must be designed so that a main portion of ultrasonic radiation is radiated at an angle φ that causes the radiation to be received at a receiving transducer. As the ultrasonic propagation path is in reality affected by the temperatures of ultrasonic wedges employed for the transducers 30, 50, as well as temperatures of the conduit walls and velocities of the ultrasonic radiation and flow velocity v, a certain ultrasonic radiation beam width is necessary for the transmitted ultrasonic radiation to reach a receiving transducer 30, 50 as appropriate. Depending on beam width and departure from the theoretical model, ultrasonic radiation may propagate not only along the assumed path 20, but also simultaneously through multiple paths with transit times that differ slightly from the expected values. Such spurious paths influence a transit time measurement accuracy that can be achieved and are especially relevant when ultrasonic transducers are mounted on an external surface of a conduit. A method described in U.S. Pat. No. 4,930,358 for improving flow measurement accuracy is therefore based on reducing an angle of directivity and thus the number of spurious ultrasonic propagation paths. Reduced angles of directivity are typically accomplished by increasing the size and coupling surface area of ultrasonic transducers employed.
U.S. Pat. No. 5,856,622 discloses an iterative method for temperature and pressure compensation in the calculation of flow velocity from transit times measured using the aforementioned conventional method. Moreover, U.S. Pat. Nos. 4,195,516, 4,930,358 and 5,280,728 disclose transducer wedge portions that are designed to allow on-line measurement of the sound velocity of the wedge material. It is found that the sound velocity in the transducer wedge is important both with respect to transit times and with respect to an angle of refraction achieved into the liquid. Disclosures in these US patents indicate different ways to compensate for temporal uncertainties due to variable transducer delay and propagation path, which the conventional method is sensitive to, but they do not propose any approach to fully avoid any of these problems. U.S. Pat. No. 4,748,857 proposes an apparatus and a method wherein a mounting distance between transducers is altered to compensate for sound velocity changes in a fluid to be characterized. Such adjustments are impractical in many applications and potentially can give rise to increased apparatus cost, extra complexity and reduced reliability.
In a U.S. Pat. No. 4,454,767, there is described an apparatus including two ultrasonic transducers with wedges which are integrated into a single clamping mechanism to ensure proper mutual positioning of the transducers when the apparatus is mounted into an outer surface of a pipe. The apparatus may enable practical installation in a clamp-on manner, but does not compensate for measurement uncertainty due to variations in temperature and fluid composition.
A coherent multi-path flow measurement system is described in U.S. Pat. No. 6,293,156, the system being based upon transmission of a high-frequency ultrasonic beam into a wall of a steam- or gas-carrying pipe. This beam is reflected in operation from an inner and outer surface of a wall of the pipe and thus impinges on an inner wall at repeated locations separately axially by a skip distance. For each such incidence, a portion of the ultrasonic energy within the pipe is radiated into a flowing medium present in the pipe, thus forming multiple discrete ultrasonic propagation paths through the medium. A plurality of ultrasonic receivers are positioned to receive ultrasonic signals transmitted along different paths, and flow velocity of the medium is found by cross-correlation of the received signals. The flow measurement system is not a transit-time flow meter and is not subject to the same uncertainties as aforementioned conventional flow meters. However, the measurement system is subject to other uncertainties, namely related to the skip distance and ultrasonic beam width. Moreover, the measurement system may operate at frequencies which are too high for multiphase flow measurements, for example as pertinent to oil industries.
As Equation 1 (Eq. 1) indicates, accurate knowledge of the sound velocity c in the fluid is important for flow velocity measurement by the conventional method. The sound velocity c is also often a sought-after parameter for characterization of the fluid, and is generally obtained by undertaking separate measurements. For example, U.S. Pat. Nos. 3,731,532, 3,783,169, 3,727,454, 3,727,458 and 4,015,470 disclose methods that employ three or four transducers to measure both flow and sound velocity. U.S. Pat. No. 5,040,415 discloses use of four transducers for measuring transit times for four paths through the fluid and therefrom infer flow velocity, temperature and pressure from the measurements. As the sound velocity of ultrasonic radiation in a fluid is generally pressure and temperature dependent, fluid characterization measurements must either be performed under the particular conditions of interest, or the temperature and pressure must also be measured and associated correction applied. Moreover, the sound velocity in a multiphase flow is strongly dependent upon fluid composition and may fluctuate rapidly as the fluid composition fluctuates. A need to add temperature and pressure measurement to conventional fluid flow measurement apparatus represents an added complexity and cost.
In U.S. Pat. Nos. 4,467,659 and 4,838,127, there are described designs for flow measuring apparatus that produce and detect Lamb guided-wave modes in a wall of a fluid-carrying conduit. The generated Lamb modes couple to the fluid flowing in the conduit so that ultrasonic signals follow paths for which measured transit times can be combined for calculating the flow velocity of the fluid. Moreover, in a U.S. Pat. No. 4,735,097, there are described ultrasonic transducers mounted onto a surface of a plate-like structure such as a fluid-carrying conduit, the transducers being operable to generate a Rayleigh-like disturbance at the remote surface of the wall. This disturbance functions as an extensive aperture which is several ultrasonic wavelengths wide in respect of an ultrasonic signal radiating into the fluid. A very short pulse is employed for generating such Rayleigh-like oscillations without exciting Lamb-modes. The aforementioned three patents are concerned with transducer design per se, and not flow velocity measurement.
From the foregoing, it will be appreciated that considerable technical effort has been devoted to develop and evolve ultrasonic flow measurement apparatus. Such effort has resulted in complex instruments which experience measurement accuracy difficulties when presented with complex flow mixtures, for example multiphase liquid/gas mixtures including particulate matter. Several approaches exist for compensating for measurement uncertainties inherent to the conventional method. However, hitherto, despite extensive effort as elucidated in the foregoing, no alternative methods have been disclosed that keep the advantages but avoid the aforementioned inherent uncertainty of ultrasonic transit-time flow measurement.